Material teknik I

1. 1
MSE 280: Introduction to Engineering Materials
Reading: Chapter 2
Atomic structure and Bonding
g p
Overview
•Electrons, protons and neutrons in atoms
(Bohr and QM models).
•IP, EA, χ, and periodic trends.
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•Bonding between atoms.
•Intermolecular forces.
•Relation to macroscopic properties.
Electrons in atoms
orbital electrons:
n = principal
quantum number
1
Nucleus: Z = # protons
Adapted from Fig. 2.1,
Callister 6e.
Electrons in discrete orbitals.
Bohr atom:
n=3 2 1
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N = # neutrons
Atomic mass A ≈ Z + N
1) electrons are particles that revolve
around the nucleus.
2) quantized angular momentum.
Quantum Mechanics:
Wave or matrix mechanics
→ Probability.

2. 2
Comparison of Bohr and QM models
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Figs. 2.2 and 2.3 from Callister 6 ed.
Atomic orbitals
dyz
dxy
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s px py pz
dz2dx2-y2dxz
• have discrete energy states (Quantized).
• tend to occupy lowest available energy state.
Electrons...

3. 3
Quantum numbers
• Principal: n = 1, 2, 3, 4…
• Angular momentum: l = 0, 1, 2, 3…, n – 1 = s, p, d, f…
s = sharp, p = principal, d = diffuse, f = fundamental
• Magnetic: ml = 0, ±1, ±2, ±3… , ±l
Determines the number of states in a given l subshell
(2l +1 total)
• Spin: ms = ±1/2
e g
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e.g.
1s n = 1, l = 0, ml = 0, ms = 1/2 n = 1, l = 0, ml = 0, ms = -1/2
2s n = 2, l = 0, ml = 0, ms = 1/2 n = 2, l = 0, ml = 0, ms = -1/2
Which atom is this? Be
Electron Configuration
- Shorthand notation to represent which states electrons
occupy in an atom (without specifying electron spin).
e.g. Carbon
1s
2s
Electron configuration: 1s22s22p2
2p
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Note
- each energy level can only hold two electrons of opposite spin
(Pauli exclusion principle).
- for degenerate levels (e.g. 2p-orbitals), each orbital is filled with
one electron before electrons are paired up.

4. 4
Electron configuration
1 electron in the s-orbital: Alkali metals
Li, Na, K, Rb…
2 electrons in the s-orbital: Alkaline earths
Be, Mg, Ca…
Filled s-orbital and 4 electrons in p-orbital: Chalcogens
O, S, Se…
Filled s-orbital and 5 electrons in p-orbital: Halogens
F Cl Br
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F, Cl, Br…
Partially filled d-orbital: Transition metals
e.g. Mn, Fe, Co…
Valence electrons determine which group atoms belong to.
Stable configuration
• have complete s and p subshells
t d t b i t
Stable electron configurations...
• tend to be inert.
Z Element Configuration
2 He 1s 2
10 Ne 1s 22s 22p 6
18 Ar 1s 22s 22p 63s 23p6
Adapted from Table 2.2,
Callister 6e.
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36 Kr 1s 22s 22p 63s 23p63d10 4s 24p6
Noble gases

5. 5
Valence electrons
2p
3s
2p
3s
Filled shell}
Valence electron
1s
2s
p
Na
1s
2s
Na+
Filled shell
leads to
stability.
}
Lose an electron
3
3p
3s
3p Filled shell}
Valence electrons
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Gain an electron
1s
2s
2p
3s
Cl
1s
2s
2p
3s
Cl-
leads to
stability.
How much energy does it require to take an electron out
of an atom?
Energy of an electron in vacuum
Energy
Valence electron
IP
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• Ionization potential (IP): Energy required to pull out a valence
electron (in vacuum).
By convention, IP is positive (i.e. need to put in energy to pull out
the electron).

6. 6
How much energy does it require to place an electron in
an atom?
Energy of electron in vacuum
Energy
Valence electrons
EA
Lowest available state
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• Electron Affinity (EA): Energy gained by putting an electron in (from
vacuum).
By convention, EA is negative (i.e. electron goes from higher energy
state in vacuum to lower energy state in atom).
How do we determine when an atom will accept an
electron or give one up?
y
EA IP
Vacuum level
• Electronegativity (χ): a measure of how likely an atom will take up or
give up an electron
EAIP +
Energy
Valence electrons
EA
Lowest available state
IP
χ
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A simple (and intuitive) definition:
-When two atoms are brought together, the atom with larger χ will have
higher electron density around its nucleus.
-Larger Δχ more ionic bond.
2
~
EAIP
x
+

7. 7
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Bonding
Primary
Ionic E
Covalent
Metallic
Secondary
Dipole-dipole
H bonds
E
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H-bonds
Dipole-induced-dipole
Fluctuating dipoles
Equilibrium bond length

8. 8
Ionic Bonding
• Occurs between + and - ions.
• Requires electron transfer.
Na (metal)
unstable
Cl (nonmetal)
unstable
electron
• Large difference in electronegativity required.
• Example: NaCl
3s
3p
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electron
+ -
Coulombic
Attraction
Na (cation)
stable
Cl (anion)
stable
Na
(χ = 0.9)
3s
3p
Cl
(χ = 3.0)
Ionic Bonding
Na (metal)
unstable
Cl (nonmetal)
unstable
electron
+Na (cation) Cl (anion)
ezz
EA
4
2
21
=+ -
Coulombic
Attraction
Na (cation)
stable
Cl (anion)
stable ro
A
πε4
Since z1 = +1 for Na+ and z2 = -1 for Cl-
r
A
r
e
E
o
A −=−=
πε4
2
Negative energy means attraction only.
Will the atoms collapse on themselves?
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No, there is also repulsive energy (e.g. e-e repulsion)
nR
r
B
E =
B and n depend on atoms involved.
In many cases n ~ 8.

9. 9
Ionic Bonding
E
r
A
E
nR
r
B
E =
Bond
energy
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r
EA −=
Equilibrium bond length
gy
Note: Other types of bonds can also be described in a similar manner
Ionic Bonding: examples
• Predominant
bonding in Ceramics
M O
NaCl
He
-
Ne
-
Ar
-
Kr
-
Xe
-
F
4.0
Cl
3.0
Br
2.8
I
2.5
Li
1.0
Na
0.9
K
0.8
Rb
0.8
H
2.1
Be
1.5
Mg
1.2
Ca
1.0
Sr
1.0
Ti
1.5
Cr
1.6
Fe
1.8
Ni
1.8
Zn
1.8
As
2.0
Cs Cl
MgO
CaF 2
O
3.5
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Give up electrons Acquire electrons
Rn
-
At
2.2
Cs
0.7
Fr
0.7
Ba
0.9
Ra
0.9
Adapted from Fig. 2.7, Callister 6e. (Fig. 2.7 is adapted from Linus Pauling, The Nature of the Chemical
Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by Cornell
University.
From Callister 6e resource CD.

10. 10
Covalent Bonding
1s 1s
HH
Molecular orbitals
• “Sharing” of electrons
• Why do some atoms want to share electrons?
• Example2: CH4
C: has 4 valence e,
needs 4 more
• Example1: H2
shared electrons
from carbon atom
H
CH4
H H
HH
Atomic orbitals
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H: has 1 valence e,
needs 1 more
Electronegativities
are same or comparable.
Adapted from Fig. 2.10, Callister 6e.
shared electrons
from hydrogen
atoms
HH
H
C
an s-orbital three p orbitals
Atomic Orbitals
atomic orbitals for carbon:
1s
2s
2p
4 valence electrons but
two different types
orbitals.
H’s on CH4 should be
equivalent.
x
y
z
px py pz
an s-orbital p
Hybridization sp3 hybridization for C in CH4
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x
y
z
1s + 2p = sp2-orbitals 1s + 3p = sp3-orbitals
60°
60°
x
y
z
y
x
y
z
1s + 1p = sp-orbitals
sp3 hybridization for C in CH4

11. 11
EXAMPLES: COVALENT BONDING
He
-
H
2 1
SiC
C(diamond)
H2O
H2
Cl2
F2
columnIVA
Ne
-
Ar
-
Kr
-
Xe
-
Rn
-
F
4.0
Cl
3.0
Br
2.8
I
2.5
At
2.2
Li
1.0
Na
0.9
K
0.8
Rb
0.8
Cs
0.7
Fr
0.7
2.1
Be
1.5
Mg
1.2
Ca
1.0
Sr
1.0
Ba
0.9
Ra
0.9
Ti
1.5
Cr
1.6
Fe
1.8
Ni
1.8
Zn
1.8
As
2.0
SiC C
2.5
Cl2
Si
1.8
Ga
1.6
GaAs
Ge
1.8
O
2.0
c
Sn
1.8
Pb
1.8
Adapted from Fig. 2.7, Callister 6e. (Fig. 2.7 is
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• Molecules with nonmetals
• Molecules with metals and nonmetals
• Elemental solids (RHS of Periodic Table)
• Compound solids (about column IVA)
0.9 p g , ( g
adapted from Linus Pauling, The Nature of the Chemical Bond, 3rd edition, Copyright 1939 and
1940, 3rd edition. Copyright 1960 by Cornell University.
% ionic character
Most bonds between two different types of atoms are somewhere in
between ionic and covalent.between ionic and covalent.
% ionic character = ]})(25.0exp[1{ 2
BA χχ −−−
χj = electronegativity of atom j
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χj electronegativity of atom j
KEY POINT: Larger electronegativity difference more ionic

12. 12
Example problem
• Order the following semiconductors from
t l t t t i imost covalent to most ionic.
1) ZnS, GaP, CuCl
2) ZnS, ZnSe, ZnO
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What’s so important about ionicity
of bonds?
• Chemical properties
NaCl (highly ionic solid) dissolves readily in water but– NaCl (highly ionic solid) dissolves readily in water but
Si (covalent solid) does not.
• Electronic properties
– Ionicity of the bonds will have a strong influence on
the band gap and other electronic properties.
• All properties of materials are largely determined
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All properties of materials are largely determined
by the types and strength of bonds between the
constituent atoms.

13. 13
Metallic Bonding
• Arises from a sea of donated valence electrons
Fixed ion cores (nuclei
+ + +
+ + +
Fixed ion cores (nuclei
and inner electrons)
“sea” of electrons
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• Primary bond for metals and their alloys.
• Large atomic radius and small IP will more likely lead to metallic bonding.
+ + + Adapted from Fig. 2.11, Callister 6e.
Secondary Bonds: Intermolecular Forces
• Dipole-dipole interaction: secondary bond between molecules with
permanent dipole moments
Van der Waals
+ - secondary
bonding
+ -
H Cl H Clsecondary
bonding
secondary bonding
-general case:
-ex: liquid HCl
-ex: polymer
Adapted from Fig. 2.14,
Callister 6e.
Adapted from Fig. 2.14,
Callister 6e.
From Callister 6e resource CD.
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• Hydrogen bonding
H
O
H
H
O H
H
O
H

14. 14
• Dipole-induced-dipole interaction: secondary bond
between molecules with permanent dipole moments
+
-
+
- +
-
• Fluctuating dipoles
H2 H2
ex: liquid H 2asymmetric electron
clouds
secondary
bonding
polar
Nonpolar
(e.g. atom)
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HH HH
H2 H2
secondary
bonding
clouds
+ - + -secondary
bonding
Adapted from Fig. 2.13, Callister 6e.
SUMMARY: BONDING
Type
Ionic
Bond Energy
Large!
Comments
Nondirectional (ceramics)Ionic
Covalent
Large!
Variable
large-Diamond
small-Bismuth
Variable
Nondirectional (ceramics)
Directional
(semiconductors, ceramics
polymer chains)
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Metallic
Secondary
large-Tungsten
small-Mercury
smallest
Nondirectional (metals)
Directional
inter-chain (polymer)
inter-molecular
From Callister 6e resource CD.

15. 15
• Bond length, r
F
F
• Melting Temperature, Tm
Energy (r)
PROPERTIES FROM BONDING: TM
• Bond energy, Eo
r
Energy (r)
unstretched length
r
larger T
smaller T m
ro
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Eo=
“bond energy”
ro
r
u st etc ed e gt
larger T m
Tm is larger if Eo is larger.
From Callister 6e resource CD.
• Elastic modulus, E cross
sectional
area A o
length, Lo
undeformed ΔLF
Elastic modulus
PROPERTIES FROM BONDING: E
• E ~ curvature at ro
ΔL
F
undeformed
deformed
ΔLF
Ao
= E
Lo
Energy
unstretched length E is larger if curvature at
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r
larger Elastic Modulus
smaller Elastic Modulus
ro
unstretched length E is larger if curvature at
ro is larger.
From Callister 6e resource CD.

16. 16
Ceramics
(Ionic & covalent bonding):
Large bond energy
large Tm
large E
small α
SUMMARY: BONDING and Materials’ properties
Metals
(Metallic bonding):
Polymers
small α
Variable bond energy
moderate Tm
moderate E
moderate α
Directional Properties
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Polymers
(Covalent & Secondary):
secondary bonding
Directional Properties
Secondary bonding dominates
small T
small E
large α
From Callister 6e resource CD.